Effects of mean stress and fibre volume content on the fatigue-induced damage mechanisms in CFRP

The effect of the load type (tension and compression) in quasi-static and of the applied mean stress in fatigue tests on the mechanical behaviour and on the damage mechanisms in unidirectional (UD) carbon/epoxy laminates has been studied in combination with the influence of fibre volume content. Results show that the fibre volume content increases the mechanical properties in tension–tension fatigue tests for all tested angles 0°, 45° and 90°. The tensile damage mechanisms of off-axis specimens depend on the fibre volume content and change from matrix cracking and matrix–fibre debonding to fibre-pull out with an increasing amount of fibres as investigated in detail in a previous work. In tension–compression tests, higher fibre volume contents are only beneficial in fatigue tests at angles of 0° and 45°. Fatigue strengths of UD 90° specimens in tension–compression tests are not significantly improved by the fibre volume content which can be ascribed to breakage of entire fibre bundles and crushed fibres on the fracture surfaces.     

A study of fibre and interface parameters affecting the fatigue behaviour of natural fibre composites

The tension–tension fatigue behaviour of different natural fibre reinforced plastics was investigated. The composites used were made of flax and jute yarns and wovens as reinforcements for epoxy resins, polyester resins and polypropylene.Fibre type, textile architecture, interphase properties, fibre properties and content were found to affect the fatigue behaviour strongly as illustrated with damping versus applied maximum load curves. It was found that natural fibre reinforced plastics with higher fibre strength and modulus, stronger fibre–matrix adhesion or higher fibre fractions possess higher critical loads for damage initiation and higher failure loads. In addition, damage propagation rates were reduced.Furthermore, unidirectional composites were less sensitive to fatigue induced damage than woven reinforced ones.     

Flexural properties of z-pinned laminates

This paper examines the effect of pinning on the flexural properties, fatigue life and failure mechanisms of carbon/epoxy laminates. Five-harness satin weave carbon/epoxy laminates were reinforced in the through-thickness direction with different volume fractions and sizes of fibrous composite pins. Microscopic examination of the laminates before flexural testing revealed that the pins caused considerable damage to the microstructure, including out-of-plane crimping, in-plane distortion and breakage of the fibres and the formation of resin-rich zones around each pin. The pins also caused swelling of the laminate that reduced the fibre volume content. Despite the damage, the pins did not affect the flexural modulus of the laminate. However, increasing the volume content or diameter of the pins caused a steady decline in the flexural strength and fatigue life, which appear to be governed by fiber rupture on the tensile side of the laminate. Property changes are discussed in terms of transitions in the dominant failure mechanisms due to the presence of pins.     

Failure modes of fibre reinforced composites: The effects of strain rate and fibre content

The many aspects of high speed response of fibre reinforced composite materials have received the attention of a large number of investigators. Nevertheless, the understanding of the mechanisms governing failure under high speed loadings remain largely unknown. The effect of rate and fibre content on failure mechanisms was investigated by viewing fractured surfaces of tensile specimens using a scanning electron microscope (SEM). Tensile tests were conducted on a woven glass/epoxy laminate at increasing rates of strain. A second laminate (with random continuous glass reinforcement) was tested in tension at varying fibre volume fractions in order to ascertain the relationship between fibre content and failure mechanisms. The results suggest a brittle tensile failure in fibres of the woven laminate. In addition, the matrix was observed to play a greater role in the failure process as speed was increased, resulting in increased matrix damage and bunch fibre pull-out. The results also indicated that increasing the fibre volume fraction increased the likelihood of a matrix dominated failure mode.     

Damage modelling of laminated carbon/epoxy composites under static and fatigue loadings

This study deals with the modelling of the behaviour of laminated carbon/epoxy composites under static and fatigue loading. The non-linear cumulative damage model developed is written on the scale of the plies. It is based on a multi-criterion approach: brittle behaviour in the fibre direction, elastoplastic damage behaviour under shear and transverse tension stress, and elastoplastic behaviour under transverse compression loading. The range of validity of the model described here is limited to the first intra-laminar macro-crack, and it does not account for the delamination processes. This first-ply failure criteria model provides a conservative approach, which should be useful in some industrial context where the highest safety is required. Static tests results obtained on plate samples are presented [±45°]_3s and hybrid glass/carbon [0,90]_s are presented: they justify the modelling adopted here. The first tension/tension fatigue test results obtained on plate samples [±45°]_3s are presented: they are used to identify the parameters of the model contributing to the shear behaviour.     

Fatigue life evaluation for carbon/epoxy laminate composites under constant and variable block loading

This paper presents the results of current research on the fatigue life prediction of carbon/epoxy laminate composites involving twelve balanced woven bidirectional layers of carbon fibres and epoxy resin manufactured by a vacuum moulding method. The plates were produced with 3 mm thickness and 0.66 fibre weight fraction. The dog bone shape specimens were cut from these plates with the load line aligned with one of the fibre directions. The fatigue tests were performed using load control with a frequency of 10 Hz and at room temperature. The fatigue behaviour was studied for different stress ratios and for variable amplitude block loadings. The damage process was monitored in terms of the stiffness loss. The fatigue life of specimens submitted to block loading tests was modelled using Palmgren–Miner’s law and taking in to account the stress ratio effect. The estimated and experimental fatigue lives were compared and good agreement was observed.     

Die Schwingfestigkeit von Bauteilen aus faserverstärkten Werkstoffen

Fatigue Strength of Components of Fibre Reinforced Materials The constant and variable amplitude fatigue strength as well as the tensile and compressive strength of a composite material with carbon/HT fibre and epoxy matrix were experimentally determined. Unnotched dogbone shaped specimens of multidirectional CFRP with 0°, ± 45° and 90° layers were used for the tests. The results of constant amplitude testing with the stress ratios R = +0.1; ?0.5; ?1.0; ?1.66; and ?5.0 are presented. The variable amplitude testing was done using a flight-by-flight loading sequence which is typical for the wing root area of a fighter airplane. The life was also calculated and the outcome was compared with the test result. The influence of loading modifications regarding the high- and the low load-cycle ranges of the load spectrum was also examined. The fatigue behaviour of the composite material was compared with the fatigue behaviour of metals.     

Fatigue characteristics of plasma treated aluminium repaired by graphite/epoxy composite patch

Abstract

In the present study, the fatigue behaviour of plasma treated aluminium patched by a graphite/epoxy composite (carbon fibre reinforced plastic, CFRP) has been investigated. The aluminium was surface treated using a dc plasma containing acetylene gas and nitrogen gas at a volume ratio of 5 : 5 for 30 s. The effect of plasma treatment on the fatigue behaviour of the aluminium/CFRP specimen was determined from fatigue testing using two different single edge notched (SEN) specimens of cracked aluminium repaired with a CFRP patch and plasma treated aluminium also repaired with a CFRP patch. The load ratio and the frequency applied in the fatigue tests were 0.1 and 10 Hz, respectively. The surfaces of the aluminium specimens were examined using atomic force microscopy (AFM) to investigate the effect of plasma treatment on the surface morphology. The results showed that plasma treated specimens exhibited almost 12% more fatigue life than untreated specimens. The surface roughness of aluminium was increased ~1.5 times by plasma treatment. The increased surface roughness improved the bonding strength between aluminium and the CFRP patch, increasing the fatigue life of aluminium patched by CFRP.     

Effects of fibre content on mechanical properties and fracture behaviour of short carbon fibre reinforced geopolymer matrix composites

Geopolymer matrix composites reinforced with different volume fractions of short carbon fibres (C_f/geopolymer composites) were prepared and the mechanical properties, fracture behaviour and microstructure of as-prepared composites were studied and correlated with fibre content. The results show that short carbon fibres have a great strengthening and toughening effect at low volume percentages of fibres (3·5 and 4·5 vol.%). With the increase of fibre content, the strengthening and toughening effect of short carbon fibres reduce, possibly due to fibre damage, formation of high shear stresses at intersect between fibres and strong interface cohesion of fibre/matrix under higher forming pressure. The property improvements are primarily based on the network structure of short carbon fibre preform and the predominant strengthening and toughening mechanisms are attributed to the apparent fibre bridging and pulling-out effect.     

The environmental fatigue behaviour of carbon fibre reinforced polyether ether ketone

The fatigue behaviour of carbon fibre/PEEK composite is compared with that of carbon/ epoxy material of similar construction, particularly in respect of the effect of hygrothermal conditioning treatments. Laminates of both materials were of 0/90 lay-up, and they were tested in repeated tension at 0° and at 45° to the major fibre axis. The superior toughness of the polyether ether ketone and its better adhesion to the carbon fibres results in composites of substantially greater toughness than that of the carbon/epoxy material, and this is reflected in the fatigue behaviour of the carbon fibre/PEEK. The tougher PEEK matrix inhibits the development of local fibre damage and fatigue crack growth, permitting a 0/90 composite with compliant XAS fibres to perform as well in fatigue as an epoxy laminate with stiffer HTS fibres. Hygrothermal treatments have no effect on the fatigue response of either material in the 0/90 orientation. The fatigue response of a cross-plied carbon/PEEK laminate in the ±45° orientation is much better than that of equivalent carbon/epoxy composites, again because the superior properties of the thermoplastic matrix.     

Characterising the loading direction sensitivity of 3D woven composites: Effect of z-binder architecture

Three different architectures of 3D carbon fibre woven composites (orthogonal, ORT; layer-to-layer, LTL; angle interlock, AI) were tested in quasi-static uniaxial tension. Mechanical tests (tensile in on-axis of warp and weft directions as well as 45° off-axis) were carried out with the aim to study the loading direction sensitivity of these 3D woven composites. The z-binder architecture (the through-thickness reinforcement) has an effect on void content, directional fibre volume fraction, mechanical properties (on-axis and off-axis), failure mechanisms, energy absorption and fibre rotation angle in off-axis tested specimens. Out of all the examined architectures, 3D orthogonal woven composites (ORT) demonstrated a superior behaviour, especially when they were tested in 45° off-axis direction, indicated by high strain to failure (∼23%) and high translaminar energy absorption (∼40 MJ/m³). The z-binder yarns in ORT architecture suppress the localised damage and allow larger fibre rotation during the fibre “scissoring motion” that enables further strain to be sustained by the in-plane fabric layers during off-axis loading.     

Improvement of fatigue life by incorporation of nanoparticles in glass fibre reinforced epoxy

The fatigue properties of glass fibre reinforced epoxy laminates modified with small amounts (0.3 wt.%) of nanoparticles (fumed silica SiO₂ and multi-wall carbon nanotubes (MWCNT)) were evaluated by means of static (90°-tensile and stepped tensile) and dynamic fatigue tests. For the MWCNT-modified matrix, the electrical conductivity was measured in situ. The addition of nanoparticles lead to increases in inter fibre fracture strength of up to 16%. More significantly, the high cycle fatigue life is increased by several orders of magnitude in number of load cycles. The increased inter fibre fracture strength could be correlated to the improved fatigue behaviour, as final failure in high cycle fatigue is strongly correlated to matrix cracks. For the MWCNT-modified composites, the state of load and damage state was monitored by conductivity measurements. A correlation between the onset of matrix cracking and increase in electrical resistivity could be drawn enabling self sensing capabilities.     

Fatigue residual strength of circular laminate graphite–epoxy composite plates damaged by transverse load

The research dealt with the relation between damage and tension–tension fatigue residual strength (FRS) in a quasi-isotropic carbon fibre reinforced epoxy resin laminate. The work was organized in two phases: during the first one, composite laminates were damaged by means of an out-of-plane quasi-static load that was supposed to simulate a low velocity impact; in the second phase, fatigue tests were performed on damaged and undamaged specimens obtained from the original composite laminates. During the quasi-static transverse loading phase, damage progression was monitored by means of acoustic emission (AE) technique. The measurement of the strain energy accumulated in the specimens and of the acoustic energy released by fracture events made it possible to estimate the amount of induced damage and evaluate the quasi-static residual tensile strength of the specimens. A probabilistic failure analysis of the fatigue data, reduced by the relative residual strength values, made it possible to relate the FRS of damaged specimens with the fatigue strength of undamaged ones.     

Influence of temperature and thickness on the off-axis behaviour of short glass fibre reinforced polyamide 6.6 – cyclic loading

This paper investigates the anisotropic properties of short glass fibre reinforced polyamide 6.6 (PA66-GF35) under tension–tension and tension–compression cyclic loading. Tensile fatigue tests were carried out on dog-bone specimens, machined out from injection-moulded plates 80 × 80 mm, of three different thicknesses t (1 and 3 mm) at three different nominal fibre orientation angles θ (0°, 30° and 90°). The tests were carried out at RT as well as at 130 °C.The Tsai–Hill failure criterion, modified to account for cyclic loading, is applied to the fatigue data for estimating the fatigue strength parameters of the material under investigation. Results are compared to the strength parameters obtained under quasi-static loading in a previous part of this work [De Monte M, Moosbrugger E, Quaresimin M. Influence of temperature and thickness on the off-axis behaviour of short glass fibre reinforced polyamide 6.6 – quasi-static loading. Composites: Part A, 2010;41(10):1368–79]. The experimental results highlight how specimen thickness remarkably affects mechanical properties: the thinner the specimen the higher will be the degree of anisotropy. Also temperature strongly reduces the fatigue strength under cyclic loading. The Tsai–Hill criterion allows for an adequate fitting of experimental data at the investigated temperatures and load ratios.     

Bolted Joints in Three Axially Braided Carbon Fibre/Epoxy Textile Composites with Moulded-in and Drilled Fastener Holes

Experimental behaviour of bolted joints in triaxial braided (0°/±45°) carbon fibre/epoxy composite laminates with drilled and moulded-in fastener holes has been investigated in this paper. Braided laminates were manufactured by vacuum infusion process using 12 K T700S carbon fibres (for bias and axial tows) and Araldite LY-564 epoxy resin. Moulded-in fastener holes were formed using guide pins which were inserted in the braided structure prior to the vacuum infusion process. The damage mechanism of the specimens was investigated using ultrasonic C-Scan technique. The specimens were dimensioned to obtain a bearing mode of failure. The bearing strength of the specimens with moulded-in hole was reduced in comparison to the specimens with drilled hole, due to the increased fibre misalignment angle following the pin insertion procedure. An improvement on the bearing strength of moulded-in hole specimens might be developed if the specimen dimensions would be prepared for a net-tension mode of failure where the fibre misalignment would not have an effect as significant as in the case of bearing failure mode, but this mode should be avoided since it leads to sudden catastrophic failures.     

Schädigungsentwicklung bei Ermüdung verschiedener CFK-Laminate

Fatigue Damage Development of Various CFRP-Laminates The behaviour of a woven carbon-fibre reinforced laminate in a balanced eight-shaft satin weave style was compared to non-woven laminates with an equivalent cross-ply lay-up (50% of the fibres in the 0° and 50% in the 90° direction) Two types of non-woven laminates were investigated consisting of continuous fibres and aligned discontinuous fibres, both produced from carbon fibre prepregs. The static strength and the fatigue behaviour was measured. Stiffness reduction was monitored during fatigue loading as a damage analogue to which the mechanisms of damage could be associated. Similarities and differences in the fatigue behaviour and damage development of the three laminates will be discussed.     

A comparison of fatigue damage detection in carbon and glass fibre/epoxy composite materials by acoustic emission

Ring specimens of four fibre composite materials, two containing carbon fibres (crfp) and two containing glass fibres (gfrp) have been monitored for acoustic emission (AE) during tension/ tension fatigue cycling.In cfrp continuous monitoring failed to provide warning of impending fatigue failure. The results are not consistent with the view that cessation of activity after initial shakedown guarantees stabilization and safe operation.In gfrp, if the activity is averaged over adequate periods, an underlying trend is revealed, short term deviations from which are attributed to the occurrence of a different damage process which determines final failure. It is shown that in the tests reported, the progressively increasing component of the activity can be related to the fatigue life. The distribution of events over the load cycle shows that a dominant part of this progressively increasing activity is associated with some stress independent process. Use of the appropriate part of the load cycle also offers the possibility of focussing on the events associated with the damage that leads to final failure.     

Experimental determination of the mode I delamination fracture and fatigue properties of thin 3D woven composites

The mode I delamination fracture toughness and fatigue strength of thin-section three-dimensional (3D) woven composite materials is experimentally determined. The non-crimp 3D orthogonally woven carbon–epoxy composites were thin (2 mm) and consequently their through-thickness z-binder yarns were inclined at a very steep angle (about 70°) from the orthogonal direction. The steep z-binder angle has a marked effect on the delamination toughening and fatigue strengthening mechanisms. Experimental testing revealed that the fracture toughness and fatigue resistance increased progressively with the volume content of z-binders. However, the steep angle caused the z-binder yarns bridging the delamination crack to deform and fail in shear and through-thickness tension, rather than in-plane tension which usually occurs in thick 3D woven composites. Mode I pull-off tests on a single woven z-binder yarn embedded within the composite revealed that the crack bridging traction load, strain energy absorption and failure mechanism were strongly affected by the steep angle.     

Post-impact compressive behaviour of low temperature curing woven CFRP laminates

The polymeric matrix in a fibre-reinforced composite serves to bind the fibres together, transfer load to the fibres and protect them against environmental attack and damage due to handling. The matrix has a strong influence on several mechanical properties of the composite such as transverse modulus and strength, impact resistance, shear properties and properties in compression. This paper describes the results of an experimental study to determine the effect of resin (matrix) on the post-impact compressive behaviour of carbon fibre woven laminates. Three new low temperature cure (50–125°C) epoxy resins are examined: an unmodified (LTM12), a rubber-modified (LTM25) and a thermoplastic toughened epoxy resin (MT8E). Note, however, that the first two are post-cured at 190°C. Velocities and impact energies were used to simulate momenta typical of low velocity impact hazards associated with aircraft in-service. Measurements of impact damage and damage growth during compression are made using ultrasonic C-scanning and penetrant-enhanced X-ray radiography techniques. For low impact energies the superior performance of the thermoplastic toughened epoxy is confirmed. Its residual compressive strength compares favourably with that obtained for high strength carbon fibre/epoxy laminates manufactured from unidirectional sheets cured at 190°C.     

Fatigue behaviour assessment of flax–epoxy composites

The present study focuses on the characterisation and evaluation of the fatigue behaviour of flax–epoxy composites. A better understanding of this behaviour allows the prediction of long-term properties to assess the viability and long-term durability of these materials. The purpose of this work is to systematically compare the tension–tension fatigue behaviour of flax fibre composites for one random mat, six textile architectures and two laminate configurations, which are used in a wide range of applications. The fibre architecture was found to have a strong effect on the fatigue behaviour, where higher static strength and modulus combinations present the best fatigue characteristics. They have a delayed damage initiation and increased fatigue life as well as a reduced damage propagation rate combined with higher energy dissipation in the early stages of fatigue loading.